Variable VHE gamma-ray emission from non-blazar AGNs

Abstract

The observation of rapidly variable very high energy (VHE) gamma-rays from non-aligned active galactic nuclei (AGNs), as reported from M87, proves challenging for conventional theoretical acceleration and emission models. We re-examine the centrifugal acceleration of particles by rotating jet magnetospheres in the vicinity of accreting supermassive black hole systems and analyze the energy constraints imposed for highly underluminous systems. Applications are presented for conditions expected to be present in the radio galaxy M87, assuming accretion onto the central black hole to occur in an advection-dominated (ADAF) mode. We show that for a highly underluminous source like M87, centrifugally accelerated electrons may reach Lorentz factors up to $\gamma \sim (10^7-10^8)$, allowing inverse Compton (Thomson) upscattering of sub-mm disk photons to the TeV regime. Upscattering of Comptonized disk photons results in a flat TeV spectrum $L_{\nu} \propto \nu^{-\alpha_c}$ with spectral index $\alpha_c \simeq 1.2$. The characteristic variability time scale is of the order $r_{\rm L}/c$, which in the case of M87 corresponds to $\simeq 1.7$ d for a typical light cylinder radius of $r_{\rm L} \simeq 5 r_{\rm s}$. Centrifugal acceleration could thus provide a natural explanation for the challenging VHE emission features in M87. Our results suggest that some advection-dominated accreting (non-blazar) AGNs could well be observable VHE emitting sources. (abridged)